Disk drive devices, or devices that use disks of various forms, including an optical disk, a magneto-optical disk, or a flexible disk, are known. Among the devices, a hard disk drive (HDD) is one of storage devices that are widespread as storage devices for computers and indispensable in the present computer systems. Moreover, because of its excellent characteristics, uses of the HDD are not limited to computer systems but are increasingly expanding to moving image recording and playing apparatuses, car navigation systems, cellular phones, removable memories used in digital cameras, and the like.
A magnetic disk used in the HDD has plural tracks formed in a concentric shape. Servo data and user data are stored in the respective tracks. A head element unit formed of a thin film element may perform data writing or data readout by accessing a predetermined area (address) in accordance with the servo data. In data readout processing, a signal read out from the magnetic disk by the head element unit is subjected to predetermined signal processing such as waveform shaping and decode processing by a signal processing circuit and transmitted to a host. Transfer data from the host is written in the magnetic disk after being subjected to the predetermined processing by the signal processing circuit in the same manner.
As described above, the respective tracks include user data areas in which the user data is stored and servo pattern areas in which the servo data is stored. Servo patterns are composed of cylinder IDs, sector numbers, burst patterns, and the like. The cylinder IDs indicate addresses of the tracks and the sector numbers indicate sector addresses in the tracks. The burst patterns have relative position information of a magnetic head with respect to the tracks.
The servo patterns are formed in plural sectors spaced apart in a circumferential direction in the respective tracks. The servo patterns of the respective sectors over all the tracks are aligned in positions (phases) in the circumferential direction. Readout of data from or writing of data in the magnetic disk is executed while a position of the magnetic head is checked according to the servo data in a state in which the magnetic disk is rotating.
The servo patterns are written in the magnetic disk in a factory before the HDD as a product is shipped. Conventional typical writing of servo patterns is performed using a servo writer as an external apparatus. The HDD with a top cover thereof removed is set in the servo writer. The servo writer positions a head in the HDD with a positioner (an external positioning mechanism). A servo pattern generating circuit writes generated servo patterns in the magnetic disk.
At present, a writing process of servo data occupies a main position in manufacturing cost of the HDD. In particular, in recent years, there is a keen competition for an increase in capacity of the HDD and, therefore, a TPI (Track Per Inch) is rapidly increasing. According to the increase in the TPI, the number of tracks increases and a track width decreases. This causes an increase in STW time and improvement of precision of the servo writer and ultimately raises the cost of the STW. In order to reduce this cost, a reduction in the cost of the servo writer, a reduction in the STW time, and the like are in progress.
For example, SSW (Self Servo Write) uses only a mechanical system of HDD itself for servo write, controls a spindle motor and a voice coil motor in an HDD from an external circuit, and writes servo patterns using the external circuit. Consequently, a reduction in cost of the servo writer is realized.
A conventional SSW method includes making use of the fact that radial direction positions of a read element and a write element of a head element unit are different (this is called read write offset), the read element performs positioning while reading out a servo pattern already written on an inner radial side or an outer radial side and the write element writes a new servo pattern in a desired track apart from the servo pattern by the read write offset.
In this way, in self propagation of the servo pattern for writing the new track, it is expected that, basically, accuracy in the radial direction of the servo pattern read is directly inherited by the servo pattern written anew. However, because of various factors that cause errors, this accuracy is deteriorated by the propagation. The deterioration in the positioning accuracy causes a deviation from an desired position of a burst pattern. This deviation is inherited at the next propagation. This process also depends on characteristics of a servo loop for causing the head element unit to follow a track. In this way, the deterioration in the accuracy is caused by a complicated mechanism involving the various factors.
In order to prevent this deterioration in the accuracy of the track follow due to the propagation, in the SSW, the following method is proposed in Japanese Patent Publication No. 08-212733 (“Patent Document 1”). For example, a case in which a track TO is now followed and a new track TN is about to be written is considered. A position error signal (PES) of the track follow at the time when the track TO is written is stored in advance. This PES is subjected to the Fourier transform and thus obtained Fourier components of PES is multiplied by a reference correction coefficient.
This correction coefficient is prepared for components with large gains (the components are usually limited to only several components with small frequencies) among frequency components of a closed loop response of a positioning servo of the head element unit. Subsequently, the components are subjected to the inverse Fourier transform to correct target values in the respective sectors at the time when the track TN is followed. Consequently, a propagation error in the radial direction is controlled.
The correction method disclosed in the Patent Document 1 includes, in the correction of the target values in the respective sectors, (1) discrete Fourier transform for the PES, (2) control of the components with large gains of the Fourier transform by multiplication of the coefficient, (3) calculation of a correction value by the discrete inverse Fourier transform, and (4) acquisition of a new target position by addition of the correction value to a target reference position. It is necessary to execute this correction processing for the respective sectors when the respective tracks are written. However, extremely large arithmetic operation power is required to repeat the Fourier transform and the inverse Fourier transform for the respective sectors. Therefore, in order to correct the target values of the respective sectors, a method different from the above method is desired.